GMC's of the 50's used a 15 lb. cap. Later when a 50/50 coolant mixture became more common a 7 and even a 4 lb were used. I don't have the books on Chev. But the bottom line is, something went wrong that it overheated. Higher pressure raises your boiling point. Steam is created when you go over the boiling point. You probably have a plugged radiator, a defective thermostat, plugged water jacket or I have even seen impellers rust off of water pumps.A radiator cap is not meant as a safety valve in case of overheating, but rather to allow water to overflow if the radiator gets over full due to expansion. Make sure you find the cause of the overheating. Damage much more serious than splitting a radiator tank seam can be done with over heating.